There are known image forming apparatuses such as copying machines, etc., provided with a cylindrical photoreceptor drum 101 as shown in FIG. 22. In such image forming apparatuses, the surface of the photoreceptor drum 101 is charged by a main charger 102, and the photoreceptor drum 101 is exposed by projecting thereon a light beam from an exposure unit 103, and the resulting electrostatic latent image is developed by a developing unit 104, and is transferred to a sheet by a transfer charger 105. Thereafter, the charge on the surface of the photoreceptor drum 101 is removed by a cleaning blade (not shown) of a cleaning unit 106, and the developed image is permanently affixed onto the sheet by a fusing unit 107 (not shown). In the described image forming process, the photoreceptor drum 101 is driven by a drive unit so as to rotate in one direction.
In Japanese Unexamined Patent Publication No. 120265/1983 (Tokukaisho 58-120265) and Japanese Unexamined Utility Model Application No. 155863/1986 (Jitsukaisho 61-155863, there is shown a drive unit wherein the photoreceptor drum 101 provided with an internal gear is rotated by a drive gear having a small diameter in engagement with the internal gear (for simplicity, the term internal gear system is used herein) as an example of the drive system for the photoreceptor drum 101. The described arrangement is superior to the arrangement where the photoreceptor drum 101 is provided with an external gear in engagement with the drive gear in the following points. That is, as a greater number of teeth of the drive gear and the driven gear (internal gear) are in engagement with one another, unstable driving condition affected by the drive pitch of the gear is less likely to occur. Additionally, as the drive gear, etc., can be formed in the inside of the photoreceptor drum 101, the drive device can be made compact, and thus the miniaturization of the image forming apparatus can be achieved.
In the described arrangement, however, various process elements such as a main charger 102, an exposure unit 103, a developing unit 104, a transfer charger 105, a cleaning unit 106, etc., are provided along the circumference of the cylindrical photoreceptor drum 101, and among these process elements, the developing unit 104 and the cleaning blade of the cleaning unit 106 in tight contact with the photoreceptor drum 101 respectively apply contact pressure onto the photoreceptor drum 101. Therefore, the photoreceptor drum 101 is deformed to some extent which causes a displacement or decentering of the axis. On the other hand, as a greater number of teeth of the drive gear and the driven gear (internal gear) are in engagement with one another as described earlier, depending on the relative position between the process elements in tight contact with the photoreceptor drum 101 and the drive gear, the distortion in torque may occur, thereby presenting the problem that a smooth rotary motion of the internal gear, i.e., the photoreceptor drum 101 by the drive gear cannot be ensured. Such unstable rotary motion of the photoreceptor drum 101, if occurred, would cause a default image. Japanese Unexamined Patent Publication No. 120265/1983 (Tokukaisho 58-120265) and Japanese Unexamined Utility Model Application No. 155863/1986 (Jitsukaisho 61-155863) fail to provide the solution to such problem.
There are known photoreceptor units through Japanese Unexamined Utility Model Application No. 155863/1986 (Jitsukaisho 61-155863) and Japanese Unexamined Patent Publication No. 120265/1983 (Tokukaisho 58-120265). In Japanese Unexamined Utility Model Application No. 155863/1986 (Jitsukaisho 61-155863), there is shown a photoreceptor unit having arrangements illustrated in FIG. 23 and FIG. 24. As shown in these figures, around both ends of a photoreceptor drum 201, plural rollers 202 are provided for supporting the photoreceptor drum 201. On the inner circumference of the photoreceptor drum 201, 203 is formed an internal gear unit 203, and a rotary motion of the photoreceptor drum 201 is actuated by a drive gear 205 in engagement with an internal gear member 204 of the internal gear unit 203.
In Japanese Unexamined Patent Publication No. 120265/1983 (Tokukaisho 58-120265), there is shown a photoreceptor unit having an arrangement shown in FIG. 25. As shown in the figure, an internal gear unit 302 is centrally situated on an inner circumference of a photoreceptor drum 301. With an internal gear member 303 of the internal gear unit 302, engaged is a gear 305 for a motor 304 which is formed in the inside of the photoreceptor drum 301.
The described conventional photoreceptor units have the following drawbacks. That is, the former photoreceptor unit requires a complicated structure for supporting a rotatable photoreceptor drum, and a smooth rotary motion of such photoreceptor drum is difficult to be ensured. On the other hand, the latter photoreceptor unit does not refer to the desirable structure of a support mechanism for the rotatable photoreceptor drum.
To solve the described problem, there has been proposed a photoreceptor unit shown in FIG. 26. The photoreceptor unit is arranged such that an internal gear unit 403 with an internal gear member 402 is fitted to the end of a photoreceptor drum 401. A rotary motion of the photoreceptor drum 401 is actuated by a drive system including a drive pinion gear 404.
The internal gear unit 403 includes a gear support member 405 formed on a face perpendicular to an axis of the photoreceptor drum 401 and a bearing member 406 centrally situated in the gear support member 405. An end portion 406a formed in an axial direction of the bearing member 406 is situated to the inside in an axial direction with respect to an end portion 402a of the internal gear member 402.
In the described arrangement, as the bearing member 406 is formed right below the internal gear member 402, the foreign substances such as powders generated by the abrasion of the internal gear member 402, etc., may drop and enter the bearing member 406, and the frictional resistance of the bearing member 406 increases, which may even damage the bearing member 406 itself. This may result in the problems of unstable rotary motion, shaking and locking of the photoreceptor drum 401 or an increase in load during a rotary motion thereof.
The internal unit 403 shown in FIG. 26 is arranged such that the length B of the teeth tip portion of the internal gear member 82 (402) (see an explanatory view of FIG. 20) is selected to be larger than the length A of the teeth bottom portion as shown in FIG. 20. For this reason, for example, the shrinkage at the portion of the length B delays in the cooling process in the resin manufacturing process. As a result, as shown in FIG. 27, the free end side of the internal gear member 402 formed perpendicular to the gear support member 405 may be deformed towards the center of the internal gear unit 403.
Such deformation occurs by the following mechanism. For example, in the case where a member which includes a thick ridge portion 502 centrally situated on a flat plate 501 is formed by an injection molding as shown in FIG. 28(a), in general, as the hardening process is delayed, the thick ridge portion 502 shrinks and is bent in the direction of an arrow in FIG. 28(b).
The maximum amount of deformation of the internal gear member 402 would be around 50-80 .mu.m. This may decrease the precision of the internal gear member 402, and the meshing error per pitch of 20 .mu.m and a total meshing error of 60 .mu.m would not be maintained within respective desirable ranges. Especially for the internal gear member 402, different from the normal flat gear, the meshing error exceeding the backlash, if occurred, would interfere the non-driving surface, and the described deformation may not be a serious problem.
Additionally, to solve such problem, the length A shown in FIG. 20 cannot be made larger in view of improving respective precision of the internal gear member 402 and the internal unit 403 in the direction of the diameter and in consideration of a possible shrinkage, etc.